Air pollution is a concern for apparatuses which burn hydrocarbon fuels with air. Gas turbines and, more specifically, microturbines, which are used to generate electricity and hot exhaust gases used in cogenerative applications are increasingly subject to air emission restrictions. These turbines are sometimes known as industrial gas turbine generators (ITGs). The air emission restrictions are imposed by governmental regulatory organizations such as the California Air Resources Board, the states of Texas and New Jersey, and other governmental bodies. These restrictions may regulate the emission of oxides of nitrogen (NOx), carbon monoxide (CO), and volatile organic compounds (VOC's). Gas turbine engine manufacturers are obliged to develop improved combustion methods and apparatus configurations to satisfy these restrictions while also satisfying turbine engine operation requirements at full-power and low-power operation.
The low emissions combustors used in ITG engines combust a premixed combination of fuel and air. The mixture of fuel and air is typically lean of the stoichiometric apportionment of fuel and air in order to limit flame temperatures and reduce gaseous emissions. By “lean” it is meant that an excess amount of air is mixed with the fuel, and not all the oxygen in the air is consumed in the reaction. When the fuel is composed of a gaseous fuel, such as natural gas, digester gas, landfill gas, syngas derived from gasification or pyrolysis processes, or other hydrocarbon gas mixtures, the gas fuel is premixed with air prior to combustion. This is commonly referred to as “lean premixed” (LP) combustion. When the fuel is liquid, such as jet fuel, diesel, kerosene, or other liquid fuel, the fuel must be both vaporized and mixed prior to combustion. This method is referred to as “lean, premixed, prevaporized” (LPP) combustion.
Both LP and LPP combustion methods are capable of combusting fuel with low levels of NOx, CO, and VOC's. The lean fuel/air mixture combusts at low gas temperatures, avoiding high-temperature regions that produce NOx. The LP or LPP are also typically designed to burn hot enough, and for sufficient residence-times, to fully oxidize carbon monoxide (CO) to carbon dioxide (CO2) and unburned hydrocarbons and other VOC's to water (H2O) and carbon dioxide (CO2).
A typical LP and LPP combustor can burn fuel with low emissions over a limited range of fuel/air mixtures. The mixture must remain lean enough to avoid the production of NOx. This lean mixture is typically close to the lean flame extinction limit, also known as the lean blowout (LBO) limit. When gas turbines are required to produce less than full power (“part-power”), the combustor typically receives less fuel, which decreases the fuel/air ratio, inducing LBO. Gas turbine combustors sometimes include a second source of fuel (pilot) which is injected into the combustor without premixing the fuel with air. The pilot fuel burns in a “diffusion” mode, where the flame front is locally controlled by the diffusion of fuel and air (oxygen) together. Diffusion flames burn at higher temperatures and produce higher levels of NOx, but permit gas turbines to operate at part-power.